This invention relates to an encoded signal transmission method and an encoded signal transmission apparatus suitable when used in the case of encoding digital signals at variable bit rate at the transmitting side, and of carrying out transmission of encoded digital signals at variable bit rate to the receiving side.
As the television broadcast of the next generation, plans or schemes to digitize moving picture signals are being developed in order to realize high quality transmission of moving picture signals. In this case, if moving picture signals are digitized as they are, data quantity becomes vast. For this reason, in order to efficiently carry out transmission thereof by way of limited transmission path, encoding (information compression) of data is required.
Meanwhile, in general, moving pictures are not steady, but pattern or motion (movement) of picture changes in point of time. Moreover, there are instances where pattern or motion (movement) at the central portion of picture image and that at the peripheral portion are different to much degree even within picture on screen. For this reason, information quantity generated in carrying out encoding at the encoder changes in dependency upon the property of such picture. In this case, it is known that when variable bit rate encoding is used, high picture quality can be obtained as compared to fixed bit rate by the same code quantity. In order to send such information at fixed transmit bit rate, transmitting buffer is prepared at the final stage of the encoder. Namely, such an approach is employed that encoded output of which quantity generated changes is temporarily stored into the transmitting buffer to read out it at a predetermined transmission bit rate to output it to the transmission path.
Moreover, in the case of carrying out transmission of plural video sequences, when time changes of information quantities of respective pictures of plural video sequences are examined, there is low probability that respective information quantities become great value at the same time. In view of the above, it is known that output data of plural encoders are multiplexed so that sum total of information quantities generated is smoothed. For this reason, in the field of broadcast/communication, plural video sequences are encoded respectively at variable bit rates to multiplex output data of those encoders to allow it to be single bit stream of fixed bit rate to carry out simultaneous transmission thereof. This method is called statistical multiplexing.
FIG. 1 shows a block diagram of system for carrying out statistical multiplexing of outputs of plural encoders as described above (hereinafter called statistical multiplexing encoder system).
The statistical multiplexing encoder system shown in FIG. 1 is composed of n number of encoders 1011xcx9c101n, transmitting buffers 1021xcx9c102n, an encoding bit rate control circuit 103, and a multiplexing circuit 104 for encoder output.
In this case, n number of video signals are delivered to respective encoders 1011xcx9c101n through respective terminals, and are inputted to the encoding bit rate control circuit 103. The respective encoders 1011xcx9c101n encode video signals inputted thereto to output encoded data, and to output encoding bit quantities of encoded data.
The encoding bit rate control circuit 103 calculates encoding bit rates R1xcx9cRn corresponding to the property of input picture images of respective encoders from n number of video signals inputted thereto and encoding bit quantities from respective encoders. At this time, control is conducted such that sum total of R1 to Rn every predetermined time becomes constant. Encoding bit rates of respective encoders 1011xcx9c101n are changed every 15 frames, for example. Transmitting buffers 1021xcx9c102n are respectively provided at succeeding stages of respective encoders 1011xcx9c101n to respectively temporarily store encoded data from the respective encoders 1011xcx9c101n into the transmitting buffers 1021xcx9c102n to absorb changes of code quantities generated. The multiplexing circuit 104 multiplexes, in a time divisional manner, plural output bit streams respectively delivered from the respective buffers 1021xcx9c102n to send out it to the transmission path as single bit stream.
Such statistical multiplexing encoder system is disclosed in detail, e.g., in Bulletin of Television Society Vol. 50, No. 7, pp. 958-964 (1996).
In statistical multiplexing encoder system as described above, there is known a method of conducting a control such that sum total of encoding bit rates every predetermined time to respective encoders becomes constant to adaptively carry out bit rate allocation.
For example, in the statistical multiplexing encoder system shown in FIG. 1, control is conducted in the state where sum total of R1 to Rn is limited so as to cause it to be maintained constant when determining Rn from encoding bit rate R1 every predetermined time in correspondence with the property of input pictures of respective encoders.
However, employment of only such limitation is insufficient as the condition for allowing input bit rate to the multiplexer to be kept stably constant and allowing output bit rate from the multiplexer to be kept stably constant. The reason thereof is that it is not guaranteed that sum total of output bit rates of buffers of respective encoders becomes constant. For example, when delay time from the time when encoding bit rate Ri has been changed at the i-th encoder 101i(i=1xcx9cn) until output bit rate Rout_i of buffer 102i changes is assumed to be xcfx84i, in the case where xcfx84i is not constant at respective encoders, even if control is conducted such that sum total of encoding bit rates R1xcx9cRn every predetermined time becomes constant, it is not guaranteed that sum total of output bit rates Rout_1 to Rout_n every predetermined time from respective buffers becomes constant.
For this reason, there were possibility that there may take place the problem that sum total of input bit rates to the multiplexer is above the capacity of the transmission path, the problem that multiplexed output bit rate may be above the capacity of the transmission path and/or the problem that buffer of the encoder or the decoder may overflow or underflow.
This invention has been made in view of the above-described problems and its object is to provide an encoded signal transmission method and an encoded signal transmission apparatus in which in the case where outputs from plural encoders for encoding digital signals at variable bit rate are multiplexed to carry out transmission at fixed (constant) bit rate, control is conducted such that sum total of input bit rates to the multiplexer and/or multiplexed output bit rate do not exceed the capacity of the transmission path, and buffer of the encoder or the decoder may not overflow or underflow.
In an encoded signal transmission method according to this invention, an approach is employed to encode a digital signal at variable bit rate to smooth change of code quantity of short time at buffer to control output bit rate from the buffer on the basis of delay time xcfx84 from the time when encoding bit rate of the encoder has been changed until output bit rate of the buffer changes in outputting bit stream at a predetermined bit rate from the buffer.
It is preferable that the delay time xcfx84 is determined by
xcfx84=B/Rmin
when minimum value of encoding bit rate of the encoder is Rmin and decoder buffer size is B.
Moreover, in the case where n (nxe2x89xa72) number of the encoders are provided to respectively provide encoder buffers at respective encoders to multiplex respective buffer outputs to carry out transmission thereof, output bit rates from the respective encoder buffers are controlled on the basis of respective delay times xcfx84i (i=1xcx9cn) from the time when encoding bit rates of respective encoders have been changed until output bit rates of the buffers change.
Also in this case, it is preferable that the respective delay times xcfx84i are determined by
xcfx84i=B/Rminxe2x80x94i
when minimum value of encoding bit rate of each encoder is Rmin_i and decoder buffer size is B.
Moreover, it is preferable to select maximum value xcfx84max of delay time xcfx84i with respect to respective encoders to change output bit rates of respective buffers after delay time (xcex94xcfx84 is fixed time of 0 or more common to the n number of encoders in this case) from the time when encoding bit rates of respective encoders are changed (xcfx84max+xcex94xcfx84).
Further, it is preferable to select maximum value xcfx84max of delay time xcfx84i with respect to respective encoders to use such a predetermined time xcex4i to satisfy
R1(t+xcex41)+R2(t+xcex42)+ . . . +Rn(t+xcex4n)=constant
when encoding bit rate at time t of the i-th (i=1xcx9cn) encoder of n number of encoders is assumed to be Ri(t) to change output bit rate of the i-th encoder after delay time of (xcfx84max+xcex4i) from the time when encoding bit rate of the i-th encoder is changed, and the predetermined time xcex4i is determined by
xcex4i=xcfx84maxxe2x88x92xcfx84i.
Further, an encoded signal transmission apparatus to which such an encoded signal transmission method is applied may be constituted.
Outputs from respective encoder buffers are sent to multiplexer, at which they are multiplexed. The multiplexed output thus obtained is caused to undergo transmission. At this time, an approach is employed to control output bit rates from respective encoder buffers on the basis of respective delay times xcfx84i, thereby making it possible to allow sum total of input bit rates to the multiplexer to be stably constant.